Cyclometallation and NN bond cleavage of 2-(arylazo)phenols by osmium. Synthesis, structure and redox properties

Abstract

Reaction of 2-arylazo-4-methylphenol, (H₂ap-R, where H₂ indicates the two protons (the phenolic proton and one phenyl proton at the ortho position of the arylazo fragment) that undergo dissociation upon complexation) with [Os(PPh₃)₃Br₂] in the presence of a base afford two types of organometallic complexes of osmium(III), viz. red [Os(PPh₃)₂(ap-R)Br] and brownish-green [Os(PPh₃)(ap-R)(N–O)] [N–O is an iminosemiquinonate ligand generated from the 2-(arylazo)phenol via NN bond cleavage]. The structures of [Os(PPh₃)₂(ap-Cl)Br] and [Os(PPh₃)(ap-Cl)(N–O)] have been determined by X-ray crystallography. In the [Os(PPh₃)₂(ap-R)Br] complexes, the ap-R ligand coordinates to osmium as a tridentate C,N,O donor ligand forming two five-membered chelate rings and the two PPh₃ ligands are trans. In the [Os(PPh₃)(ap-R)(N–O)] complexes, osmium is bound to one PPh₃, one ap-R ligand coordinated as a tridentate C,N,O donor ligand and one iminosemiquinonate ligand. The [Os(PPh₃)₂(ap-R)Br] complexes are paramagnetic (low-spin d⁵, S = 1/2) and show anisotropic EPR spectra at 77 K. The [Os(PPh₃)(ap-R)(N–O)] complexes are diamagnetic and show sharp ¹H and ¹³C NMR signals. In acetonitrile solution all the complexes display intense charge-transfer transitions in the visible region. Cyclic voltammetry on these complexes in acetonitrile solution shows an osmium(III)–osmium(IV) oxidation positive to SCE and an osmium(II)–osmium(III) reduction negative to SCE. Two irreversible oxidations are also displayed by all the [Os(PPh₃)(ap-R)(N–O)] complexes in the range 1.10–1.76 V vs. SCE.